Refractor Theme and ActiveHours contexts into an AppContext

[DeshmukhChinmay]

User Story:

As a Developer, I want to refactor the current 'Theme' and 'ActiveHours' context into a single AppContext so that it is easier to maintain all of the global states.

Acceptance Criteria:

1. The respective fields from 'ThemeContext' and 'ActiveHoursContext' must be present in the new 'AppContext'.
2. The relevant front-end component tests should be changed to incorporate this new change.

Depends on #193